Field-free or anechoic chambers have been in wide use for measurement of various characteristics of antennas, tests of measuring instruments of electromagnetic field intensity, measurement of the radiation of disturbing waves, and the like. In order to completely remove reflection of radio waves in the chamber, such a field-free chamber is so constructed as to be covered with a wave absorber on the surfaces of walls, ceilings and floors of an electromagnetic wave-shielding or insulating chamber.
Ordinarily employed wave absorbers are ones, which are made of composite resin foams obtained by a method such as impregnation of a dielectric loss material, such as carbon, or coating of the material onto foam beads. Thereafter, the resin foams are shaped in the form of a solid pyramid, wedge, polygon or corrugation. When a radio wave enters an absorber, this type of pyramid or wedge-shaped wave absorber gradually changes an impedance from its surface so as not to reflect the wave, thereby realizing good absorption properties over a wide-band frequency range.
However, the absorption properties are not satisfactory within a low frequency range. Accordingly, usual practice is to use, in combination, ferrite tiles having good absorption properties within a narrow-band frequency range.
Examples of the resin foam-based wave absorber containing a dielectric loss material include solid pyramid-shaped, wedge-shaped, polygonal or corrugated articles, which are made of composite materials obtained by uniformly distributing a dielectric loss material, such as carbon, in resin foams, such as polystyrene foams, urethane foams, polyethylene foams and the like, or coating the dielectric loss material onto foam beads.
However, since the wave absorber made of a resin foam is combustible in nature, a radio wave with great electric power is continuously forced into the absorber, whereupon it generates heat from the inside thereof with the great possibility that the absorber takes fire and burns. Thus, there is a demand for fireproofing of the absorber.
Moreover, the wave absorbers made of polyurethane foams, polyethylene foams and the like resin foams are so soft that when processed in the form of a pyramid, its sharp tip ends may hang down. In addition, they are liable to absorb moisture therein, with the attendant problem on their quality that they are poor in shape retention and durability.
On the other hand, polystyrene foams are not satisfactory with respect to their wave absorption properties, coupled with another problem that they are brittle. Further, the wave absorbers made of these resin foams are dark in color and poor in design properties, thus leaving the problem that they give a physiological pressure to a person in charge of measurement.
In order to cope with such problems as set out above, Japanese Laid-open Patent Application No. Hei 6-314894 proposes a wave absorber made of a phenolic resin foam, which is semi-incombustible or incombustible.
The wave absorber made of a phenolic resin foam is made according to a mode molding method wherein a carbon-containing phenolic resin is charged into a mold inserted with an incombustible, light-colored paper cap, followed by foaming and curing, thus substantially solving the above-stated problems.
Although wave absorbers shaped in the form of a solid pyramid, wedge or polygon have been set forth hereinabove, there is also proposed a hollow pyramid-shaped resin foam wave absorber (Japanese Laid-open Patent Application No. Hei 4-44300).
This absorber is one wherein a carbon-containing urethane resin foam is processed in the form of a hollow pyramid. The absorber aims at improving impedance matching with ferrite tiles at a bottom surface and also improving wave absorption properties, not at solving the above-mentioned problems on fireproofing, durability and shape retention.
On the other hand, there are disclosed techniques relating to solid or hollow pyramid-shaped wave absorbers made of inorganic fibers (Japanese Patent Publication Nos. Sho 64-8480, Hei 1-45237 and Japanese Laid-open Patent Application No. Hei 3-99496).
Such wave absorbers are made of composite materials obtained by incorporating dielectric loss materials, such as carbon, into glass wool or ceramic wool by impregnation and processed in the form of a pyramid. Although the absorbers are satisfactory with respect to fireproofing and absorption properties, the compositing and processing procedures are complicated, thus involving design and cost problems.